It's funny that the PS1 got so famous for 3d games, when its 'GPU' was entirely 2d.

I guess the main thing the console brought to the table that made 3d (more) feasible was that the CPU had a multiplication instruction?

A little more than just a multiplication instruction (the 68000, used in, say, the Sega Mega Drive, had one of those too). Have a look at https://www.copetti.org/writings/consoles/playstation/, and in particular, read about the GTE - it offered quite a bit of hardware support for 3D math.

Also, even though it didn't handle truly 3D transformations, the rasterizer was built for pumping out texture mapped, Gouraud shaded triangles at an impressive clip for the time. That's not nothing for 3D, compared to an unaccelerated frame buffer or the sprite/tile approach of consoles past.

It's not just a multiplication instruction. The CPU is equipped with a fixed-point coprocessor to accelerate the most common computations in 3D games, the geometry transformation engine [1], capable of carrying them out much faster than the CPU alone could. For instance, the GTE can apply a transformation matrix to three vertices and project them in 23 cycles, while the CPU's own multiplier takes up to 13 cycles for a single multiplication and 36 (!) for a division. Combined with a few other "tricks" such as a DMA unit capable of parsing linked lists (which lets the CPU bucket sort polygons on the fly rather than having to emit them back-to-front in the first place), it allowed games to push a decent number of polygons (typically around 1-3k per frame) despite the somewhat subpar performance of the cache-less MIPS R3000 derivative Sony chose.

If you have some basic familiarity with C, you can see both the GTE and the Z bucket sorting of GPU commands in action in the cube example I linked in the parent comment.

[1]: https://psx-spx.consoledev.net/geometrytransformationengineg...